Charge coupled device (CCD) and complementary metal oxide semiconductor (CMOS) cameras are being developed with higher resolution and faster frame rates. Both such goals can conflict in the design and operation of a CCD because the increase in pixels needed to get higher resolution means each row or column of pixels will take longer to serially clock out. Higher speeds demand that the pixels for each frame clock out quicker, not take longer to complete.
So the multitap CCD structure was developed that splits the image frame into two or more areas that are clocked out in parallel. For example, a frame can be divided into left and right halves and the pixels in each half serially clocked out to the nearest side edge. Alternatively, rows can be divided into even and odd, and separately clocked into even/odd row buffers.
Two problems result in practice when trying to use multitap CCD imagers. First, there will be subtle differences in the gain and black levels for each group of separately clocked out pixels. When the image is reconstructed, the eye will easily recognize any differences as shade banding. Second, small mismatches or incompatibilities in the imager to the framegrabber can interfere with the accurate restitching of the image frame sections back together. The restitching can have an extra row or column of pixels, or one can be missing. This problem is usually only present when mating a new camera to a new framegrabber. Once the details of the match have been settled between the camera and frame grabber manufacturers, the restitching problem is permanently solved. But it can appear again with minor revisions to the design or manufacturing of either the camera or the framegrabber.